Proceedings of Pile 2013, June 2-4th 2013 STRESS AND ... · PLACEM ORING S • – risch Building...

ST

RE

SS

A

ND

D

IS

PL

AC

EM

EN

T M

ON

IT

OR

IN

G A

UG

ER

P

IL

ES

– M

. D. L

aris

ch

www.pilingcontractors.com.au Building the Foundations of Australia

Proceedings of Pile 2013, June 2-4th 2013

STRESS AND DISPLACEMENT MONITORING

OF AUGER DISPLACEMENT PILES

M.D. Larisch1, M. Arnold2, M. Uhlig2, E. Schwiteilo2, D.J. Williams1 and A. Scheuermann1

1. ABSTRACT:

Auger displacement piles have been used for decades worldwide as foundation elements for structures and embankments. The system has become increasingly popular and very successful in recent years as it can achieve high production rates and spoil creation is minimal. The advancement of an auger moves and compacts the ground laterally during penetration, which can result in increased shaft friction of the completed pile. However, this technique has the potential to damage already completed piles, as well as adjacent structures or underground services, due to lateral or vertical soil displacements during pile construction.

The behaviour of the soil surrounding the auger during penetration, extraction and concrete pumping is not well understood to date. As a result, designs have been either too conservative or pile settlements have been above the specified design criteria and excessive, resulting in damage.

The paper describes auger displacement behaviour in fine-grained soils. A fully instrumented test site in Brisbane, Australia, has monitored soil behaviour (ground stresses and displacements) during the penetration and extraction of two auger different displacement pile types. The field results are compared with Finite Element model predictions, enabling them to be validated and calibrated.

Keywords: Displacement, displacement piles, Finite Element model, heave, pile monitoring, stresses, soil interaction

2. INTRODUCTION AND SCOPE

Auger displacement piles (ADP) are a rotary drilling technique that can be used to construct concrete piles and columns. During the penetration of the auger, the soil is displaced laterally into the surrounding ground, and the spoil created by ADP installation is minimal. Concrete is pumped through the hollow stem of the auger and the piling methodology is very similar to Continuous Flight Auger (CFA) piling. Due to high production rates, the technique can be very economical, which has led to its increased global usage during the last two decades. The typical ADP installation process is described in Figure 1 (after Bottiau, 1998) and below:

• Set up auger at pile position and install cap to close concrete outlet at auger tip.

• Install auger by rotating clockwise and applying vertical pull down force.

• Drill auger to design depth; the displacement body of the auger pushes the soil cut by the auger tip into the surrounding ground.

• Pump concrete through hollow auger stem and extract auger while rotating clockwise, always maintaining concrete pressure positive and auger embedded in fresh concrete.

• Install reinforcement into fresh concrete, if required.

ST

RE

SS

A

ND

D

IS

PL

AC

EM

EN

T M

ON

IT

OR

IN

G A

UG

ER

P

IL

ES

– M

. D. L

aris

ch


Fig. 1 Installation process of ADP (after Bottiau, 1998)

Depths of up to 30 m can be achieved with standard piling equipment. Different auger types and shapes are available, and typical diameters range from 200 to 550 mm. Skin friction and end bearing behaviour of different piles can vary, depending on the soil conditions, and the geometry and shape of the auger (Vermeer 2008, Larisch et al. 2012).

Soil behaviour during the entire construction process has, particularly in cohesive soils, not been investigated in detail. ADPs have the potential to damage adjacent structures or freshly cast piles due to displacement effects and heave, as has been observed on many projects.

The theoretical background of these phenomena has not been investigated in detail. The scope of this research project, led by The University of Queensland (UQ) in collaboration with Piling Contractors, is to investigate the changes in stresses and displacements in fine-grained soil during the installation of ADPs.

UQ entered into a cooperation with the Technical University of Dresden, Germany, in 2010 to develop a suitable numerical model for the penetration and extraction of a typical ADP in fine-grained soil, using a hypo-plastic constitutive model for clay (Larisch et al. 2013), and to validate the numerical model by field test results to be carried out in Lawnton, QLD, Australia.

3. FIELD TEST SITE

In February 2011, two cone penetration tests (CPT) and two undisturbed, continuous soil samples were taken to bedrock at the proposed test pile locations. The soil samples were taken about 500 mm from the CPTs in order to get a close correlation between CPT values and the soil profile. Index values were determined in the laboratory for each of the different soil layers encountered and these are summarised in Table 1.

ST

RE

SS

A

ND

D

IS

PL

AC

EM

EN

T M

ON

IT

OR

IN

G A

UG

ER

P

IL

ES

– M

. D. L

aris

ch


Table 1: Typical soil profile, index values and soil classification of Lawnton clays

Soil description

Stiff, light brown ocher clay

Stiff, grey clay, inclusions

Stiff, grey, sandy clay, inclusions

Depth (m) 0.00 to - -1.50 to - -6.20 to -

1.50 6.20 7.20

Fines content 79 91 57

< 15 mm (%)

Liquid Limit 43 55 44

(LL) (%)

Plastic Limit 25 21 19

(PL) (%)

Plasticity 18 32 25

Index (PI)

(%)

Classification Medium High Medium

DIN18196 plasticity plasticity plasticity

Classification Low High Low

USC plasticity plasticity plasticity

For this research project, only the top three soil layers are relevant. The underlying gravel with clayey inclusions has no significance to the research as the proposed test piles were to be founded 4.00 m below the original ground level, within the clay layers. The scope of this research is the investigation of auger displacement pile behaviour in fine-grained soil and the authors wanted to ensure that the piles would “float” in Lawnton clays, without any influences from the gravel layer below. Typical cone penetration test (CPT) profiles for the field test site are displayed in Figure 2.

Fig. 2 Typical CPT at the test field in Lawnton, QLD, Australia

ST

RE

SS

A

ND

D

IS

PL

AC

EM

EN

T M

ON

IT

OR

IN

G A

UG

ER

P

IL

ES

– M

. D. L

aris

ch


The second clay layer, which was defined as grey clay, is the material in which the ADPs were to be founded. Characteristics of this grey clay layer are assumed to be critical for the ADP performance.

Laboratory oedometer tests and consolidated undrained (CU) triaxial tests were carried out on undisturbed and remoulded soil specimens to determine the basic five hypo-plastic soil parameters for use in the FE model (Larisch et al. 2013).

4. NUMERICAL MODEL

For the numerical simulation of the penetration and extraction of an ADP, the Finite Element (FE) code Abaqus Standard was used (Larisch et al. 2013). Hypo- plastic soil behaviour for fine-grained soil (clays) after Mašín (2005) was implemented using a UMAT routine. An axisymmetric, two-dimensional FE model was developed to simulate the pile construction process. The model greatly simplifies the real geometry and construction process. Due to the complexity of the process, the model does not take auger rotation, soil cutting, soil transport, soil disturbance or soil compaction into account.

The ADP auger is modelled as a cone-shaped rigid body, with a 60 degree cone angle and a total auger height of 1.5 m, representing a typical ADP auger (lower screw section). To avoid excessive mesh distortion at the beginning of the penetration process, the cone is partly pre-installed into the soil and the soil and the cone are modelled to be in full contact.

The expected deviatoric stress field of an auger displacement pile during the installation process in Lawnton clay is displayed in Figure 3 and the expected horizontal soil displacements during this process are shown in Figure 4.

Fig. 3 Expected deviatoric stress field (von Mises stress in kPa) of ADP to be installed in Lawnton clays modelled using Abaqus Standard

In the past (Cudmani 2001, Henke 2010), a particular method called the “zipper technique” has been used

successfully to model pile penetration into a soil continuum. A smooth rigid tube with a diameter d = 1 mm is discretised at the axis of penetration. The cone-shaped, rigid ADP body slides over the rigid tube and separates the soil from the tube. The cone establishes contact with the soil and is able to deform the meshed continuum, thus simulating penetration and the resulting soil displacement. The surface-to-surface contact between the penetrating

ST

RE

SS

A

ND

D

IS

PL

AC

EM

EN

T M

ON

IT

OR

IN

G A

UG

ER

P

IL

ES

– M

. D. L

aris

ch


object (ADP auger) and the surrounding soil is based on the master-slave principle. The friction coefficient between the deformable soil and the rigid piling auger is assumed to be tan (φc/3), based on Coulomb’s friction law.

The diameter of the displacement auger head is modelled to be 450 mm and the penetration depth into the soil continuum is taken to be 4.00 m. Pile installation is modelled using constant penetration and extraction rates of 0.03 m/s. However, since the constitutive model is rate-independent, the rates are of no impact. Soil behaviour is assumed to be undrained during penetration (constant volume), as penetration occurs too rapidly to allow substantial drainage.

Fig. 4 Expected horizontal soil displacement (in meters) of ADP to be installed in Lawnton clays modelled using Abaqus Standard

5. FIELD TESTS

The field test site is located at Lawnton, Queensland, Australia close to the South Pine River. A 300 mm thick working platform was installed on the topsoil surface to ensure safe operations during pile installation. The platform comprised crushed rock with a maximum aggregate size of 75 mm. All pile locations, CPT, dilatometer (DMT) and spatial time domain relectiometer (TDR) locations were pre-excavated or pre-drilled through the working platform to minimise disturbance from displaced rocks during pile installation.

Two test piles were installed, both auger displacement piles 450 mm in diameter. One test pile was installed with a progressive displacement auger, referred to as “test pile C”. The second test pile was installed with a rapid displacement auger and is referred to as “test pile D”. More details about the different auger types are presented in the following section. Another aim of the research project is to compare the behaviour of different auger types in similar ground conditions using similar installation parameters, hence the use of different auger types.

The piles were installed with a centre-to-centre distance of 4.50 m between the piles. This distance was assumed to be sufficient to ensure that the installation of one pile would not influence the stress and displacement state at the other pile. The piles were installed using a Casagrande C30 piling rig, as shown in Figure 5.

ST

RE

SS

A

ND

D

IS

PL

AC

EM

EN

T M

ON

IT

OR

IN

G A

UG

ER

P

IL

ES

– M

. D. L

aris

ch


Fig. 5 Piling rig with a rapid displacement auger in preparation to install test pile D

The installation of both piles was recorded using the on-board computer of the piling rig and Jean Lutz software. Displacement piling is a blind process and it is important to ensure a constant penetration rate by monitoring changes in the torque readings of the drilling rig. Higher torque readings combined with normal penetration rates can indicate stiffer soil layers and it is advisable to carry out a test drill close to a CPT location in order to calibrate the drill parameters. For the test site, it was expected that the torque reading would increase linearly to a depth of about 2.00 m below the working platform level. The CPT shows a peak cone resistance at this level. Below this level, the torque reading should be constant, as auger penetration should occur as fast as possible. The installation records are displayed in Figure 6 and it can be observed that the energy input to install the piles is almost identical for both test piles, as torque, rotation, and penetration rates are almost similar. The lifting rate is not critical for the soil penetration process and indicates the extraction rate of the auger during concrete placement.

Fig. 6 Pile installation record for test piles C and D show very similar installation rates and energy input

IN SITU MONITORING

ST

RE

SS

A

ND

D

IS

PL

AC

EM

EN

T M

ON

IT

OR

IN

G A

UG

ER

P

IL

ES

– M

. D. L

aris

ch


CPT and DMT tests were carried out before and after the installation of the two test piles. Both tests provide some measure of the stress state in the soil before and after the installation of the test piles.

Spatial time domain reflectiometer (Spatial TDR) pressure sensors (Scheuermann and Huebner, 2009) were used for the first time to measure stresses in the ground during pile installation, continuously over a predefined depth of 5.00 m, covering the length of the pile to 1 m below the pile toe, and not only at a single point as for an earth pressure cell. The TDRs were assembled on site and installed around the test piles, as shown in Figure 7 and Table 2. The analysis of the TDR results is still ongoing and is not the subject to this paper. The working principle, assembly, installation and measurement results of the TDR for this research project will be presented in another publication (see also Scheuermann et al, 2009).

For the monitoring of horizontal soil displacements, a series of inclinometer tubes was installed around each test pile, as shown in Figure 5. The tubes were installed and grouted into 100 mm diameter holes, drilled by a subcontractor. Three sensors were lowered down each tube before and after the test pile installation to measure lateral soil displacements before and after the installation of both test piles. Measurements were taken every 100 mm over a depth of 6.00 m. It was important to measure potential changes below pile toe level as the FE model indicates stresses up to 1.0 m below the proposed pile toe.

In summary, a series of instruments was located around each test pile, as shown in Figure 7. The inclinometer tubes and TDR’s were installed and maintained in place. The CPTs and DMTs were carried out at the pile location before pile installation and outside the pile after completion of the test piles. The configuration, as shown in Figure 7, is similar for both test piles C and D.

Fig. 7 Location and direction of different monitoring equipment from the proposed test piles (not to scale)

Table 2: Distance of different monitoring devices from centre of test piles C and D of diameter d, location as per Figure 7

Distance to centre of pile

Target (mm)

Actual (mm)

Difference (mm)

CPT 1 0.0 200 200 0

CPT 2 1.0 d 450 440 -10

DMT 1 0.0 200 200 0

DMT 2 1.0 d 450 455 5

Inc. 1 1.0 d 450 465 15

Inc. 2 1.25 d 563 548 -15

Inc. 3 1.5 d 675 670 -5

TDR 1 0.72 d 325 305 -20

TDR 2 1.0 d 450 420 -30

TDR 3 1.5 d 675 605 -70

For the measurement of vertical soil displacements, survey pegs were used and measurements were taken before and after the test pile installation to observe soil heave around the piles.

ST

RE

SS

A

ND

D

IS

PL

AC

EM

EN

T M

ON

IT

OR

IN

G A

UG

ER

P

IL

ES

– M

. D. L

aris

ch


The augers

For the installation of the test piles, two different auger types were used:

• Progressive displacement auger for the installation of one test pile (pile C).

• Rapid displacement auger for the installation of one test pile (D).

In order to understand the general working principle of ADPs, it is important to be familiar with the basic principles of piling auger mechanics. Detailed descriptions of various screw auger models and the most accepted theories can be found in the literature (Viggiani 1993, Fleming 1995, Slatter 2000) and are beyond the scope of this paper. However, it is important to understand the influence of auger geometry on the stresses and displacements created in the soil, as well as the installation parameters and, potentially, the pile load capacities. All screw auger theories and models are essentially based on three basic auger actions:

• soil cutting,

• soil transport, and

• soil displacement.

Depending on the auger shape, geometry, and the main installation parameters (penetration rate, torque, auger rotations), the influence of the three auger actions is different in granular and fine-grained soils.

The augers shown in Figures 8 and 9 are all defined as full-displacement piling augers and generally fall in the group of long displacement auger systems (Larisch et al. 2012), which all have the same basic geometrical components, as shown in Figure 8 and Table 3:

Fig. 8 Basic components of long displacement augers used for the installation of pile C (left) and pile D (right)

The different basic geometry of both auger types is also shown and sketched in Figure 9. Progressive and rapid displacement augers are both designed with longer flighted sections and the lower auger sections are used for cutting and transporting the soil to the displacement body of the auger. The counter screw sections, located above the displacement body, re-displace soil that has collapsed into the cavity behind the auger during the extraction process. The auger geometry of both augers seems similar, and visually both augers seem comparable. However, one auger is a progressive displacement auger, as the diameter of the lower auger section progressively increases towards the displacement body. During penetration, soil is displaced progressively along the lower auger section and finds it peak at the location of the displacement body.

The overall height of the rapid displacement auger is about 500 mm greater than the height of the progressive displacement auger, due to the longer lower screw section of the rapid displacement auger. Both augers have

ST

RE

SS

A

ND

D

IS

PL

AC

EM

EN

T M

ON

IT

OR

IN

G A

UG

ER

P

IL

ES

– M

. D. L

aris

ch


identical outer diameters (450 mm), but the taper from the displacement body to the smaller inner diameter at the auger base is different. The progressive displacement auger is tapered over three pitches (about 750 mm) and the rapid displacement auger is tapered over only two pitch heights (about 500 mm), which results in a more rapid displacement process for the former auger.

Table 3: Typical dimensions of progressive and rapid displacement augers (DA)

Auger Details Progressive DA

Rapid DA

Outer Diameter (flights 450 mm 450 mm

and displacement body)

Inner Diameter (inner 250 mm 250 mm

tube at the bottom)

Height:

Lower Section 1,000 mm 1,500 mm

Displacement Body 1,000 mm 1,000 mm

Counter Screw Section 500 mm 500 mm

Total 2,500 mm 3,000 mm

Height Flight Pitch 250 mm 250 mm

Bottom Flight Pitch 100 mm 100 mm

For rapid displacement augers, the displacement body has a larger diameter than most sections of the inner auger stem (the flights have the same diameter as the displacement body). Soil displacement occurs rapidly at the displacement body and only minor displacements are expected below it. Potential soil loosening might occur (in granular soil) along the lower section of the auger, if penetration rates are too slow and not optimised for the ground conditions encountered.

Fig. 9 Progressive displacement auger (left) was used for the installation of pile C – for pile D the rapid displacement auger type was used (right)

ST

RE

SS

A

ND

D

IS

PL

AC

EM

EN

T M

ON

IT

OR

IN

G A

UG

ER

P

IL

ES

– M

. D. L

aris

ch


6. TEST RESULTS

CPT and DMT

For measurements after pile installation, the CPT was located 450 mm (1.0 d) from the centre of the pile. The tests were carried out typically 1-2 days after pile installation. All figures in this section display the stress changes in the soil after test pile installation.

Pile C (Progressive Displacement Auger)

Fig. 10 Pile C (progressive displacement auger) – Changes in base resistance and sleeve friction (CPT)

Significant stress changes for both cone resistance Qc and sleeve friction Fs occurred in the top 2.5 m below working platform level, indicating stress reductions after pile installation in this depth range. Ignoring a peak reading close to the surface, which could have been caused by a displaced fill boulder, the peak reduction is around 2.00 m below existing platform level with a value of -4.0 MPa (-2.0 MPa on average for this region) for Qc and about -200 kPa for Fs (average of -100 kPa), respectively.

The depth range from 2.50 to 4.00 m below working platform level shows increased stresses after pile installation, of about +1.0 MPa on average (+2.0 MPa at the peak) for Qc, and +80 kPa on average (+200 kPa at the peak) for Fs.

In the region of the pile toe, both graphs show some stress reduction between 200 m and 300 mm above and below the pile toe averaging -1.0 MPa for Qc and about - 40 kPa for Fs, which indicates soil distortion. The stresses up to 1.0 m below the pile toe increase by the same magnitude as in the depth range from 2.5 to 4.0 m below the surface.

ST

RE

SS

A

ND

D

IS

PL

AC

EM

EN

T M

ON

IT

OR

IN

G A

UG

ER

P

IL

ES

– M

. D. L

aris

ch


Fig. 11 Pile C (progressive displacement auger) – Changes in undrained shear strength (DMT)

The post-installation undrained shear strength, estimated by the DMT, confirms the results of the CPT for test pile C. The top 2.5 m showed a reduction in the average shear strength of about -50 kPa (peak reduction of -85 kPa). The depth range from -2.5 to -4.0 m indicates an increase in the average undrained shear strength of about +30 kPa (peak increase of +50 kPa). The region around the pile toe showed a strength reduction of about -10 kPa (on average) and the soil below the toe region indicated an increased strength of up to +50 kPa about 0.5 m below the pile toe. All results are summarised in the last section.

Pile D (Rapid Displacement Auger)

Fig. 12 Pile D (rapid displacement auger) – Changes in base resistance and sleeve friction (CPT)

ST

RE

SS

A

ND

D

IS

PL

AC

EM

EN

T M

ON

IT

OR

IN

G A

UG

ER

P

IL

ES

– M

. D. L

aris

ch


The stress changes after pile installation for test pile D, installed with a rapid displacement auger, appear slightly different to the stress changes around test pile C. The stress reduction in the top 2.5 m is not as marked, and for Qc the changeover point from reduced to increased stresses is located at about 0.5 m deeper at

3.00 m below the working platform surface. Both, Qc and Fs show a “zig-zag” response in the upper 2.5 to 3.0 m depth, with both positive and negative stress changes. However, the stress reduction is less significant and the magnitude is only 60% compared to pile C, showing a peak in Qc of -2.8 MPa (average of -1.6 MPa) and a peak in Fs -120 kPa (average of about -40 kPa).

The depth range below this zone to above the pile toe shows comparable stress changes to pile C for Qc (peak of +1.9 MPa and average of about +0.8 MPa) and reduced, but positive stress changes for Fs (peak of +105 kPa and average of about + 60 kPa).

The stress reduction around the toe of pile D extends to 1.0 m below pile toe level. The average value for Qc (-0.8 MPa) and Fs (-40 kPa) are almost similar for both pile types. Below this region of stress reduction, stresses increase again. However, as this region is deeper than1.0 m below pile toe level, it is assumed not to be due to the effects of the pile.

Fig. 13 Pile D (rapid displacement auger) – Changes in undrained shear strength (DMT)

The undrained shear strength, estimated by the DMT, also confirms the results of the CPT for test pile D. The strength reduction within the upper 3.0 m is more significant for pile D and shows peak values of up to - 160 kPa (average about -50 kPa). Increased strength can be observed over the lower 1.0 m section of the pile reaching an average of +30 kPa and peak of + 50 kPa, very similar to that for pile C. The region below pile toe level shows reduced shear strength as well, more than 50% lower than for pile C for both the average (-15 kPa) and peak (-30kPa) readings. The region below the toe experienced a strength increment of about +55 kPa on average (+105 kPa peak). This region is located about 0.5 m below the toe of the pile, which corresponds to the observations made at pile C (progressive displacement auger). All results are summarised in the last section.

7. PILE DISPLACEMENTS

Horizontal Displacements

Horizontal displacement changes were measured with inclinometers before and after pile installation. The horizontal displacements measured using inclinometers are shown in Figures 13 and 14, using the convention positive

ST

RE

SS

A

ND

D

IS

PL

AC

EM

EN

T M

ON

IT

OR

IN

G A

UG

ER

P

IL

ES

– M

. D. L

aris

ch


displacements away from the pile. Unfortunately, a few tubes were damaged during pile installation and only two comparable readings at 1.25 d from the centre of each test pile are available for a realistic displacement comparison.

For test pile C, maximum horizontal displacements of about 26 mm were measured about 2.5 m below the working platform. These displacements decrease almost linearly over the next 1.5 m depth to 20 mm at pile toe level. At 1 m below pile toe level, displacements are only 6 mm and the increased values measured at 6.0 m depth could be the result of a measurement error or local geological conditions.

Fig. 13 Pile C (progressive displacement auger) – Change of horizontal displacements 338 mm from the edge of the pile shaft or 563 mm from the centre of the pile

The inclinometer-based horizontal displacement measurements for pile D are shown in Figure 14. At about 2.5 m below the working platform surface, maximum displacements of about 30 mm were measured. These displacements decrease almost linearly over the remaining 1.5 m of the pile length to 26 mm at pile toe level. At 0.5 m below pile toe level, displacements are only 6 mm and reduce linearly to zero at 5.5 m depth.

ST

RE

SS

A

ND

D

IS

PL

AC

EM

EN

T M

ON

IT

OR

IN

G A

UG

ER

P

IL

ES

– M

. D. L

aris

ch


Fig. 14 Pile D (rapid displacement auger) – Horizontal displacements 338 mm from the edge of the pile shaft or 563 mm from the centre of the pile

8. SURFACE DISPLACEMENTS

Surface displacements were measured for both piles using survey pegs. For both test piles, soil vertical heave and horizontal displacements were observed during the installation of the piles.

Around both test piles a circular heave zone was observed with a radius of about 1.0 m from the edge of the piles. The height of the soil heave at the edge of the piles was about 250mm above the working platform level (the soil was pushed upwards by 250 mm) and the slope of the heave cone decreased more or less linearly to zero at the edge of the heave zone, as shown in the simplified sketch in Figure 15.

The volume of the displaced soil above the working platform level was about 0.44 m3, which is about 10% higher than the volume of the pile shaft over the top 2.5 m and could indicate dilatancy. This approach needs further refinement as dilatancy and soil loosing due to the drilling and displacement effect were not taken into account. On the other hand, the depth of the heave zone might be deeper than 2.5 m as the stresses and displacements were measured not right at the edge of the pile shaft, but about a pile diameter away from the edge of the shaft.

ST

RE

SS

A

ND

D

IS

PL

AC

EM

EN

T M

ON

IT

OR

IN

G A

UG

ER

P

IL

ES

– M

. D. L

aris

ch


Further research is required to investigate the failure mode and pattern of soil heave in fine grained soils produced by displacement augers. It is important to state that the shape of the auger seems not to have an influence on the volume of soil heave.

CONCLUSIONS For both auger test piles, significant stress changes occurred during the pile installation process. Tables 4 and 5 summarise the stress changes at the different sections of the piles. The horizontal soil displacement curves 1.25 diameters from the centre of the piles show comparable values for both piles.

The installation records in Figure 6 show that the external energy input from the piling rig is almost similar for both piles. Consequently, the stress changes observed during the installation process are solely caused by the different shape of the augers. Over the upper 2.5 m of the piles, significant stress reductions occurred for both piles, which indicate highly disturbed ground conditions caused by the displacement piling process. The authors conclude that the pile installation caused a circular failure mode around the piles. With ongoing penetration of the auger, different slip lines were created, pushing the soil upwards, similar to a base failure. Maximum horizontal displacements at 2.5 m depth also indicate that the final slip line could be located at around that depth; soil heave volumes (including dilatancy) seem comparable to the pile shaft volume to 2.5 m depth. The progressive displacement auger performs worse than the rapid displacement auger, and the reductions in stress and strength are up to 50% higher. This could be evidence that the rapid displacement auger performs more soil cutting and soil disturbance as well as less displacement during the penetration of the lower auger section. Over the upper 2.5m of the pile shaft the stress changes are lower with this auger type as the disturbed soil cannot be (re)compacted as effectively as with progressive displacement augers. The strength changes for progressive augers are smaller as the soil will not be disturbed and distorted as much as with rapid displacement augers.

Over recent years, the first authors’ experience with various soil displacement piling projects in Germany and Australia, indicates that cracks in unreinforced displacements piles and columns installed in cohesive soils usually occur in the top 1.5 m section of the pile (assuming 1 m pile spacing c/c), which is supported by the findings of this research. Soil heave is commonly known as a potential risk for auger displacement piling, but the mechanism of heave in auger displacement piling in fine-grained soil has not been investigated in detail to date. The data from the test piles at Lawnton clearly show the presence of heave; however, further research is required to define details about the failure mode, the dependence on soil types, and the influence of auger type.

Table 4: Pile C – Summary of maximum and average stress changes in Qc, Fs and Su at different depths

0 to -2.5m -2.5 to - 4.0 m Pile toe region -4.5 to - 6.0 m

Qc (MPa)

(peak)

-4.0 +2.0 -2.0 +5.0

Qc (MPa)

(average)

-2.0 +1.0 -1.0 +2.0

Fs (kPa) (peak)

-200 +200 -60 +180

Fs (kPa)

(average)

-100 +80 -40 +100

Su (kPa)

(peak)

-85 +50 -20 +65

Su (kPa)

(average)

-50 +30 -10 +30

ST

RE

SS

A

ND

D

IS

PL

AC

EM

EN

T M

ON

IT

OR

IN

G A

UG

ER

P

IL

ES

– M

. D. L

aris

ch


Table 5: Pile D – Summary of maximum and average stress changes in Qc, Fs and Su at different depths

0 to -2.5m -2.5 to - 4.0 m Pile toe region -4.5 to - 6.0 m

Qc [MPa]

(peak)

-2.8 +1.9 -1.2 -1.8

Qc [MPa]

(average)

-1.6 +0.8 -0.8 -1.0

Fs [kPa] (peak)

-120 +105 -80 +120

Fs [kPa] (average)

-40 +60 -40 +80

Su [kPa] (peak)

-160 +50 -30 +105

Su [kPa] (average)

-50 +30 -15 +55

Increased stresses and strength below the heave zone and above the pile toe were observed for both pile types and were caused by the pile installation process.

The progressive displacement auger seems to more effectively increase the stresses around the pile shaft in this region (by better displacement action) than the rapid displacement auger.

At the pile toe, both augers reduced the stresses and strengths in the ground (soft toe), to different degrees. The rapid displacement auger seemed to have more influence in regard to the depth and magnitude of stress reduction, which could be expected due to the longer lower screw section and the increased soil cutting and transport actions in this region. Potential reductions in pile base capacities caused by rotation of the auger without sufficient penetration need to be considered in pile design, particularly for rapid displacement augers. The stress reductions at the pile toe are less significant and not as deep for progressive displacement augers; however a potential reduction in the base capacity also needs to be considered.

The rapid displacement auger causes about 15% more horizontal displacement at 1.25 d from the centre of the pile than the progressive displacement auger, while vertical displacements are almost similar. It must be noted that the measured displacements might be conservative and that the “true” horizontal displacements might be larger (the FE model predicts about 60 mm lateral displacement). The inclinometers tubes are made of PVC and were grouted in the soil, providing a higher stiffness than the original soil. Consequently, the “true” horizontal displacements will be higher than the measured data, which nevertheless give a good indication about the shape and the minimum magnitude of the displacements in the ground. Two out of six inclinometer tubes were destroyed during the tests, due to excessive soil displacements, and measurements could not be taken after the pile installation. Further research is required to develop measurement devices that can monitor soil displacements more accurately. However, the measured results provide a good first indication about the shape of horizontal displacement curves.

The FE model does not allow for any soil distortion or disturbance from the cutting, transport and displacement actions throughout the entire installation process. The FE model greatly simplifies the installation process and no account was taken of soil heave and the failure pattern in the top 2.5 m of the pile. Similarly, the stress reduction at the pile base could not be modelled by the FE code, due to the complexity of the problem. Furthermore, the FE model assumes pre-excavated soil conditions at the surface, to allow full contact between the auger and the surrounding soil prior to penetration.

This assumption is not correct and does not model the heave phenomena realistically.

The authors recommend taking the measured in situ stresses (CPT sleeve friction) from -2.5 to -4.0 m into account when comparing the field values with the output data of the FE model. The numerical model predicts deviatoric stresses of about 75 kPa at a distance of 338 mm (0.75 pile diameters) from the edge of the pile. The FE model shows good agreement with the measured average sleeve friction changes of +80 kPa (<10%) for the progressive

ST

RE

SS

A

ND

D

IS

PL

AC

EM

EN

T M

ON

IT

OR

IN

G A

UG

ER

P

IL

ES

– M

. D. L

aris

ch


displacement auger and over-predicts the average stress changes after pile installation by 20 kPa (+30%) for the rapid displacement auger. However, the model provides a good indication about the stresses in the ground for progressive displacement augers in Lawnton clays. Additional research is recommended in this area to further develop numerical simulations to model soil behaviour during displacement piling using different augers. The authors believe that the development of high performance computers and improved FE codes in the next few years will greatly enhance research efforts in this area.

9. FURTHER RESEARCH

The test field results presented in this paper provide evidence that different ADP augers cause different stress changes in similar ground conditions. However, the results also open new opportunities for further research in the field of ADP in fine-grained soil conditions.

It is critical to determine the influence of heave on the load capacities of auger displacement piles. The reduced stresses and shear strengths over the upper 2.5 m of the piles can be critical, particularly for short piles and columns, as the magnitude of reduced shaft capacity in this section is unclear. Usually, auger displacement piles are assumed to have increased shaft capacities due to the displacement effect along the entire pile shaft (including the upper section). The test results provide evidence that this is not true for the top 2.5 m of the piles installed in Lawnton clays.

The authors see the need to carry out pile load tests in order to investigate the influence of auger shape and geometry on pile capacity and to correlate with the auger shape and installation parameters. In particular, the influence of heave on pile shaft capacities (positive or negative skin friction) is of great interest, especially for the design of rigid inclusions (Simon and Schlosser 2006; Plomteux and Lacazidieu 2007; Wong and Muttuval 2011 ). Research in this field is ongoing.

The installation of Continuous Flight Auger (CFA) test piles in similar ground conditions would give further data about the behaviour of different auger types in Lawnton clays. CFA piles are defined as non- displacement piles and the potential occurrence of heave and the amount of stress and displacement changes as well as load capacities in Lawnton clay, compared to auger displacement piles, would be of immense interest.

The investigation of the shape and general failure mechanism of heave is another area which requires additional research, including the influence of soil parameters and classification data on the failure pattern.

The analysis of the TDR data from the tests described in this paper is ongoing. The results will be published separately.

Furthermore, future trials in different soil conditions will provide evidence of whether or not the behaviour of auger displacement piles can be generally formalised and predicted for fine-grained soil conditions other than Lawnton clays. The authors suggest carrying out field tests prior to construction in order to get a good understanding of the stress changes, displacements and load capacities of auger displacement piles in fine- grained soils.

10. ACKNOWLEDGEMENTS

The authors thank the Group of Eight (Go8) and the DAAD (German Academic Exchange Organisation) for funding the research collaboration between The University of Queensland and the Technical University of Dresden.

Thanks are also extended to the ARC Linkage scheme and industry sponsors Piling Contractors, Golder Associates and Insitu Geotech Services for their ongoing support of the research project.

The further development of the Spatial TDR technology is supported by a recently granted Queensland Science Fellowship awarded to A. Scheuermann.

ST

RE

SS

A

ND

D

IS

PL

AC

EM

EN

T M

ON

IT

OR

IN

G A

UG

ER

P

IL

ES

– M

. D. L

aris

ch


11. REFERENCES

• Bottiau M. and Meyus I.A. (1998) “Load testing at Feluy test site: Introduction of the Omega B* pile”. Proceedings of the 3rd Int. Geotechnical Seminar on Deep Foundations on Bored and Auger Piles. W. F. van Impe and W. Haegeman. Ghent, Belgium, Balkema, A.A., Rotterdam, Brookfield, 1: 187-199.

• Cudmani, R. O. (2001). “Statische, alternierende und dynamische Penetration nicht bindiger Boeden”. Publications of the Institute of Soil Mechanics and Rock Mechanics, University of Karlsruhe (Germany), Issue 152.

• Fleming, W. G. K. (1995). “The understanding of continuous flight auger piling, its monitoring and control.” Proc. Instn Civ. Engrs Geotech,: 157 - 165.

• Henke S. (2010) “Influence of pile installation on adjacent structures”. International Journal for Numerical and Analytical Methods in Geomechanics, 34(11): 1191-1210.

• Larisch M., Nacke E., Arnold M.., Williams D.J. and Scheuermann A. (2012) “Load Capacity of Auger Displacement Piles”, Proceedings of the International Conference on Ground Improvement and Ground Control, 30th October – 2nd November 2012, Wollongong, Australia.

• Larisch M., Williams D.J. and Slatter J.W. (2013) “Simulation of Auger Displacement Pile Installation”, Int. Journal of Geotechnical Engineering, ANZ2012 Special Edition, (to be published in July 2013).

• Mašín D. (2005) “A hypo-plastic constitutive model for clays”. International Journal for numerical and analytical methods in Geomechanics, 29(4), 311-336. Plomteux, C. and Lacazedieu, M. (2007). Embankment Construction on Extremely Soft Soils using Controlled Modulus Columns for Highway 2000 Project in Jamaica, Proceedings of the 16th Southeast

• Asian Geotechnical Conference, Kuala Lumpur.

• Scheuermann, A. and Huebner, C. (2009), On the feasibility of pressure profile measurement, IEEE - Instrumentation and Measurement Magazine, 58, 467-474.

• Scheuermann, A. et al (2009), Spatial time domain reflectometry and its application for the measurement of water content distributions along flat ribbon cables in a full-scale levee model, Water Resour. Res., 45, W00D24, doi:10.1029/2008WR007073.

• Simon, B. and Schlosser F. (2006). Soil reinforcement by vertical stiff inclusions in France. Symp. Rigid Inclusions in difficult subsoil conditions, 11-12 mayo, Mexico, 22p.

• Slatter, J. W. (2000). The fundamental behaviour of displacement screw piling augers. Department of Civil Engineering. Melbourne, Monash University. PhD thesis.

• Vermeer P. (2008) “Screw piles: construction methods, bearing capacity and numerical modelling“, Bautechnik 85, Heft 2.

• Viggiani, C. (1993). “Further experiences with auger piles in Naples area.” Proceedings Deep Foundations on Bored and Auger Piles II. Balkema, Rotterdam: 445 - 455.

• Wong, P. and Muttuvel, T. (2011). Support of Road Embankments on Soft Ground using Controlled Modulus Columns. Proc. Int. Conference on Advances in Geotechnical Engineering, Perth, 621- 626.

Proceedings of Pile 2013, June 2-4th 2013 STRESS AND ... · PLACEM ORING S • – risch Building...

Documents

Transcript of Proceedings of Pile 2013, June 2-4th 2013 STRESS AND ... · PLACEM ORING S • – risch Building...